The present invention is based on Japanese Patent Application Nos. 2002-41876 and 2002-90124, each content of which is incorporated by reference.
1. Field of the Invention
The present invention relates to a developing device for use in an image forming apparatus, such as copping machines, printers and the like, the developing device serving to develop an electrostatic latent image formed on an image bearing member.
2. Description of the Related Art
Heretofore, various developing devices have been used in the image forming apparatuses, such as copying machines, printers and the like, for developing the electrostatic latent image formed on the image bearing member.
There have been known developing devices of a two-component development system using a developer containing a carrier and a toner, and those of a mono-component development system using a developer containing the toner alone or free from the carrier.
The developing devices of the mono-component development system include those of a contact development system wherein a toner bearing member is disposed in contact with the image bearing member, and those of a non-contact development system wherein the toner bearing member opposes the image bearing member via a predetermined gap therebetween in a development region.
The developing device of the contact development system features an excellent reproduction of the electrostatic latent image formed on the image bearing member because the electrostatic latent image is developed by way of physical contact between the toner and the image bearing member. Unfortunately, the toner particles also adhere to a non-imaged area not containing the electrostatic latent image and hence, the resultant image suffers fogging.
Therefore, it is a general practice to suppress the toner adhesion to the non-imaged area by, for example, differentiating moving velocities of the image bearing member and the toner bearing member.
This approach, however, involves a problem that a surface of the image bearing member is worn due to the contact with the toner bearing member and hence, the developing device cannot accomplish stable image formation.
On the other hand, an example of the developing device of the non-contact development system is shown in FIG. 1.
In this developing device, a toner xe2x80x98txe2x80x99 in a main body of a developing device 1 is moved toward a toner bearing member 3 by means of a feed member 2 so as to be held on a surface of the toner bearing member 3, which is rotated to convey the toner xe2x80x98txe2x80x99.
A regulating member 4 is pressed against the surface of the toner bearing member 3 conveying the toner xe2x80x98txe2x80x99 to a development region where the toner bearing member 3 opposes an image bearing member 10 via a predetermined gap xe2x80x98dxe2x80x99 therebetween. The regulating member 4 thus abutted regulates the amount of toner xe2x80x98txe2x80x99 held on the surface of the toner bearing member 3 while triboelectrifying the toner xe2x80x98txe2x80x99.
Subsequently, the toner bearing member introduces the regulated and triboelectrified toner xe2x80x98txe2x80x99 into the development region where the toner bearing member opposes the image bearing member 10 via the predetermined gap xe2x80x98dxe2x80x99. A developing bias source 5 applies an alternating voltage for applying an alternating electric field between the toner bearing member 3 and the image bearing member 10. The electrostatic latent image defining an imaged area of the image bearing member 10 is developed with the toner xe2x80x98txe2x80x99 supplied from the toner bearing member 3.
In this case where the regulating member 4 is pressed against the surface of the toner bearing member 3 to regulate the amount of toner xe2x80x98txe2x80x99 to be conveyed to the development region, the toner xe2x80x98txe2x80x99 is subjected to such a great load due to a contact pressure from the regulating member 4 that the toner xe2x80x98txe2x80x99 layer on the surface of the toner bearing member 3 is cracked to produce fine particles. The fine particles are gradually accumulated to be fused to the surface of the toner bearing member 3, entailing a problem that the resultant image suffers density variations.
Therefore, the conventional developing device employs the toner bearing member 3 which includes a conductive substrate 3a formed of a metal roller, and an elastic layer 3b formed over the conductive substrate and including an elastic material, such as rubber, containing a conductive material, such as carbon black. Such a toner bearing member reduces the load on the toner xe2x80x98txe2x80x99 due to the contact pressure from the regulating member 4, thereby preventing the toner xe2x80x98txe2x80x99 layer from being cracked.
Unfortunately, the toner bearing member 3 formed with the elastic layer 3b on its surface has the following problem. Since the conductive material such as carbon black, is not properly dispersed in the elastic material so that the elastic layer 3b suffers varied resistances. The varied resistances of the elastic layer lead to variations in the alternating electric field applied between the toner bearing member 3 and the image bearing member 10 and hence, the resultant image suffers density variations.
More recently, absolution to this problem has been proposed wherein a resistance control layer of high resistance is overlaid on the elastic layer, as disclosed in JP-T-2964821 (JP-A-6-264919).
However, with the use of the toner bearing member having the resistance control layer of high resistance overlaid on the elastic layer, the variations of the alternating electric field applied between the toner bearing member and the image bearing member cannot be reduced adequately. Particularly, in a case where a toner of fine particles is employed to form a fine image of high quality, resultant image still suffers the density variations.
In the conventional developing devices, it is a common practice to interpose a spacer (not shown) between the toner bearing member 3 and the image bearing member 10 such that the toner bearing member 3 and the image bearing member 10 may oppose each other via a predetermined gap therebetween. The spacer ensures a constant gap between the toner bearing member 3 and the image bearing member 10 in opposing relation.
In this approach however, the gap between the toner bearing member 3 and the image bearing member 10 opposing each other in the development region may be varied because of the variations of forming precisions or fixing precisions of these members 10, 3 or because of the wear or deformation of the spacer. This leads to varied magnitudes of the electric field applied between the toner bearing member and the image bearing member and hence, the resultant image suffers the density variations.
According to the conventional developing devices, therefore, a developing bias voltage applied between the toner bearing member 3 and the image bearing member 10 is increased in the peak-to-peak value of an AC voltage so as to cause a sufficient amount of toner xe2x80x98txe2x80x99 to jump from the toner bearing member 3 to the image bearing member 10, thereby suppressing the density variations.
In this case where the developing bias voltage is increased in the peak-to-peak value of the AC voltage, however, a potential difference between a surface potential of the image bearing member 10 and a peak value of the developing bias voltage is increased so that current leakage occurs between the toner bearing member 3 and the image bearing member 10. The current leakage detrimentally produces noises in the resultant image.
According to the state of the art, therefore, the developing bias voltage is properly controlled in the following manner. First, the current leakage is produced by varying the developing bias voltage applied between the toner bearing member 3 and the image bearing member 10, while a density sensor (not shown) senses the amount of toner xe2x80x98txe2x80x99 caused by the leakage to adhere to the image bearing member 10. Then, the developing bias voltage is set to a proper value based on the sensed amount of toner
Unfortunately, the above density sensor is expensive so that the developing device is increased in costs. In addition, the density sensor cannot detect current leakage occurred at place other than an area sensed by the sensor. Accordingly, it is impossible to set the developing bias voltage to such a proper value at all times as to prevent the occurrence of leakage.
It is an object of the invention to provide a solution to the above problems encountered by the developing device including the toner bearing member holding the toner on its surface for conveyance of the toner to the development region where the toner bearing member opposes the image bearing member via the predetermined gap therebetween; the regulating member pressed against the surface of the toner bearing member for regulation of the amount of toner conveyed to the development region; and the developing bias source for applying the alternating electric field between the toner bearing member and the image bearing member.
Specifically, a first object of the invention is to prevent the regulating member pressed against the toner bearing member for regulation of the amount of toner from cracking the toner layer on the toner bearing member to produce fine toner particles.
A second object of the invention is to provide for a simple and proper control of the developing bias voltage in order to obviate the occurrence of leakage between the toner bearing member and the image bearing member despite the errors of the gap or the like between the toner bearing member and the image bearing member.
A developing device according to a first aspect of the invention comprises a toner bearing member holding a toner on its surface for conveyance of the toner to a development region where the toner bearing member opposes an image bearing member via a predetermined gap therebetween; a regulating member pressed against the surface of the toner bearing member for regulation of the amount of toner conveyed to the development region; and a developing bias source for applying an alternating electric field between the toner bearing member and the image bearing member, and is characterized in that the toner bearing member includes a conductive substrate formed with an elastic layer, an intermediate layer and a surface layer on the surface thereof respective volume resistances xcfx811, xcfx812 and xcfx813 of which layers satisfy a condition xcfx812xe2x89xa6xcfx811xe2x89xa6xcfx813, the toner bearing member having an arithmetic average surface roughness in the range of 0.8 to 2.5 xcexcm, and that the toner has a volume-average particle size in the range of 3 to 8 xcexcm.
A developing device according a second aspect of the invention comprises a toner bearing member holding a toner on its surface for conveyance of the toner to a development region where the toner bearing member opposes an image bearing member via a predetermined gap therebetween; a regulating member pressed against the surface of the toner bearing member for regulation of the amount of toner conveyed to the development region; a developing bias source for applying an alternating electric field between the toner bearing member and the image bearing member; a leakage generator varying a leakage detection voltage applied between the image bearing member and the toner bearing member for production of leakage between the image bearing member and the toner bearing member; and a leakage detector unit for detecting the leakage based on current flowing between the image bearing member and the toner bearing member.
These and other objects, advantages and features of the invention will become apparent from the following description thereof taken in conjunction with the accompanying drawings which illustrate specific embodiments of the invention.